Catabolism of platelet-activating factor and its acyl analog. Differentiation of the activities of lysophospholipase and platelet-activating-factor acetylhydrolase.

Recent investigations have shown the presence of 1-acyl-2-acetyl-sn-glycero-3-phosphocholine, i.e. the acyl analog of platelet-activating factor (PAF), in unstimulated tissues as well as its formation along with platelet-activating factor upon stimulation of a variety of cells. We demonstrate here that this acyl analog of PAF can be catabolized by purified lysophospholipases I and II from bovine liver with near stoichiometric formation of 2-acetyl-sn-glycero-3-phosphocholine. Lysophospholipase II also deacetylated PAF to lysoPAF and evidence is presented to show that this is an intrinsic activity of this enzyme. This suggested that some lysophospholipases may contribute to intracellular inactivation of PAF by deacetylation. Anion-exchange chromatography of rat liver cytosol confirmed this possibility. However, similar experiments with rat kidney cytosol and rat and human platelet cytosol clearly separated lysophospholipase activities without PAF acetylhydrolase activity from specific PAF acetylhydrolases not having lysophospholipase activity. Thus, lysophospholipases are clearly involved in the metabolism of the acyl analog of PAF and in some tissues, such as liver, may even contribute to abolishing the biological activity of PAF through deacetylation.